Walk-through metal detectors (WTMDs) comprising an array of transmitter coils and an array of detector coils are well known and widely used for screening of personnel at secure locations such as airports, prisons, government buildings and the like. WTMDs typically operate using coupling between pairs of coils, providing a multi-zone system with a coil pair for each zone, each pair operating as an independent metal detector. In conventional systems, the transmitter coils are typically located in one panel whereas the receiver coils are located in an opposite panel, wherein the two panels comprise a walk-through portal or “hallway”.
While such systems are capable of detecting the presence of metal objects, they are limited in their ability to discriminate between different types of metal objects or accurately locate metal objects on the subject. Although there have been attempts to produce imaging metal detectors and even tomographic metal detectors that can address these concerns, these systems are limited due to the poor quality of the images that they produce. In general, such imaging systems do not produce images with sufficient quality to reliably discern the shape of the object. In addition, these conventional metal detection systems may not produce good results because they attempt to represent the metal object with a two-dimensional response. It should be appreciated by those of ordinary skill in the art that a metal object has an inherent three dimensional response that is not taken account by employing a simple two-dimensional approach.
The limitations of currently available metal detectors are well known, such as the low sensitivity to low conductivity, non-magnetic metals, e.g. titanium, and false positives caused by innocuous objects, which, in turn, result in longer queues at checkpoints and borders. In recent times, X-ray backscatter imaging techniques and systems and some millimeter-wave scanning methods and systems are becoming more widely deployed. However, these are high performance, high cost systems which are only suited to specific screening applications and there are issues, such as negative public perceptions regarding radiation exposure and/or an invasion of privacy, relating to their use.
Therefore, what is needed is a metal detection system that is capable of characterizing and locating the position of hidden objects by combining spectroscopic, tomographic and ultra-wide band techniques.
There is also a need for a new generation of electromagnetic screening equipment for detecting metallic objects which addresses the limitations of prior art, and also has minimal impact on the environment.